This disclosure relates to a system for reducing emissions in a power generation system, which utilizes hydrogen enriched fuel gas for emissions abatement in a gas turbine exhaust.
Air pollution concerns worldwide have led to stricter emissions standards. These standards regulate the emission of oxides of nitrogen, unburned hydrocarbons (UHC), and carbon monoxide (CO) generated as a result of gas turbine engine operation. In particular, nitrogen oxide is formed within a gas turbine engine as a result of the high combustor flame temperatures during operation.
The use of hydrocarbon fuels in a combustor of a fired turbine is well known. Generally, air and fuel are fed to a combustion chamber where the fuel is burned in the presence of the air to produce hot flue gas. The hot flue gas is then fed to a turbine where it cools and expands to produce power. By-products of the fuel combustion typically include environmentally harmful toxins, such as nitrogen oxide and nitrogen dioxide (collectively called NOx), CO, UHC (e.g., methane and volatile organic compounds that contribute to the formation of atmospheric ozone), and other oxides, including oxides of sulfur (e.g., SO2 and SO3).
There are two sources of NOx emissions in the combustion of a fuel. The fixation of atmospheric nitrogen in the flame of the combustor (known as thermal NOx) is the primary source of NOx. The conversion of nitrogen found in the fuel (known as fuel-bound nitrogen) is a secondary source of NOx emissions. The amount of NOx generated from fuel-bound nitrogen can be controlled through appropriate selection of the fuel composition, and post-combustion flue gas treatment. As with all cyclic heat engines, higher combustion temperature means greater efficiency. However, a problem caused by the higher combustion temperatures is the amount of thermal NOx generated. Thermal generated NOx is an exponential function of the combustor flame temperature and the amount of time that the fuel mixture is at the flame temperature. Each air-fuel mixture has a characteristic flame temperature that is a function of the air-to-fuel ratio (expressed as the equivalence ratio, φ) of the air-fuel mixture burned in the combustor. Thus, the amount of thermal NOx generated is based on the residence time, pressure, and the equivalence ratio of a particular air-fuel mixture. The equivalence ratio (φ) is defined by the following ratio: φ=(mf/mo)actual/(mf/mo)stoichiometric, where “mo” is the mass of the oxidizer and “mf” is the mass of the fuel.
The rate of NOx production is highest at an equivalence ratio of 1.0, when the flame temperature is equal to the stoichiometric, adiabatic flame temperature. At stoichiometric conditions, the fuel and oxygen are fully consumed. Generally, the rate of NOx generation decreases as the equivalence ratio decreases (i.e., is less than 1.0 and the air-fuel mixture is fuel lean). At equivalence ratios less than 1.0, more air and therefore, more oxygen is available than required for stoichiometric combustion. This results in a lower flame temperature, which in turn reduces the amount of NOx generated. However, as the equivalence ratio decreases, the air-fuel mixture becomes very fuel-lean and the flame will not burn well, or may become unstable and blow out. When the equivalence ratio exceeds 1.0, there is an amount of fuel in excess of that which can be burned by the available oxygen (fuel-rich mixture). This also results in a flame temperature lower than the adiabatic flame temperature, and in turn leads to significant reduction in NOx formation, however fuel is wasted making such a system costly and inefficient.
Prior art power generation systems use hydrogen enriched streams in the gas turbines to reduce NOx generation through reduced flame temperatures and increased operability. Hydrogen generation can be costly, however, and these power generation systems sometimes operate at less than optimal efficiencies. What is needed is a method for reducing NOx emissions in power generation systems through the use of a hydrogen enriched stream, while lowering the cost of production of both the power and the hydrogen, thereby leading to improved gains in system efficiency and operability.
Accordingly there remains a need for an improved power generation system using hydrogen rich fuel gases that can abate gas turbine emissions without suffering a loss in process efficiency.